In 1997 Tonge was elected to Parliament, serving Richmond Park from 1997 to 2005. While in the Commons she was Liberal Democrat spokesperson on International Development from 1997 to 2003, and then the spokesperson for children from 2003 to 2004.[5]

In 2002, in the Creationism – Evolution debate in the UK, she asked Prime Minister Tony Blair if he was "happy to allow the teaching of creationism alongside Darwin's theory of evolution in state schools". The Prime Minister replied that he supported a "diverse school system", triggering a row over the issue.[6]

In 2003 Tonge and Labour Party Member of Parliament Oona King visited the Gaza Strip and compared Gaza Strip living conditions for Palestinians to conditions Jews suffered in the Warsaw Ghetto during World War II.[7] Tonge said "You are almost getting a situation like the Warsaw ghetto. People can't get in or out. They can't work, they can't sell anything. There is this gradual squeeze,” as well as "It was an apartheid system." Tonge recommended economic sanctions be applied against Israel and that European Union or United Nations troops be sent to Gaza.[8] Tonge and King said an Israeli soldier wielding a grenade confronted them when they tried to leave the Gaza Strip.[7]Yad Vashem museum chair Avner Shalev criticized Tonge and King for making the comparison with conditions under Nazi rule.[9]

In January 2004 Tonge was asked to step down as children's spokeswoman by Liberal Democrat leader Charles Kennedy after saying of Palestiniansuicide bombers: "If I had to live in that situation – and I say that advisedly – I might just consider becoming one myself." Tonge refused to apologise: "I was just trying to say how, having seen the violence and the humiliation and the provocation that the Palestinian people live under every day and have done since their land was occupied by Israel, I could understand..."[10][11]

Tonge's daughter died from accidental electrocution in 2004[12] and Tonge retired from the House of Commons at the 2005 general election.[5]

From 8 January 2009 to 12 February 2010 Tonge was the Liberal Democrat Spokesperson for Health. She is very strongly pro-stem cell research and has stated so in the House of Lords.[17] She also is pro-choice and has supported " Patient (Assisted Dying)" laws in the United Kingdom.[18]

On 12 January 2009, Tonge asked in the House of Lords about investigations into alleged Israeli war crimes in the December 2009 – January 2010 Gaza War, saying:

"Is the Minister aware that Mrs Pillay, the new UN High Commissioner for Human Rights, has spoken of war crimes being committed in Gaza? Will the Government, therefore, show leadership and call for the immediate—and I mean immediate—establishment by the United Nations Security Council of an independent fact-finding commission to Palestine to investigate all breaches of international law?"[19]

In March 2009 Tonge joined a six-person delegation of British politicians which met with Hamas political leader Khaled Meshaal in Syria. Tonge told an interviewer that their goal was to force the British government to talk to Hamas and pressure the United States to do likewise. She stated "You don't make peace by talking to your friends. You make peace by talking to your enemies," and said she knew that her meeting with Meshaal might be considered illegal and lead to her arrest. "That is one of the risks that you take," she said.[21][22]

During the 2008–2009 Gaza War Tonge spoke at a London rally against the war, saying: “It is a disgusting, obscene outrage what is going on in Gaza.” She said that eventually activists must look to "a full trade embargo and boycotting everything Israeli”.[23]

On 1 February 2010 The Palestine Telegraph, of which Tonge was a patron, published an article which claimed that an Israel Defense Forces emergency aid hospital in Haiti (after the 2010 Haiti earthquake) was secretly harvesting organs and selling them on the black market, based on a YouTube video using video broadcast on Hezbollah's Al-Manar television with no cited evidence and there being no organ removal portrayed in the video.[24][25][26][27] On 11 February The Jewish Chronicle published an interview in which Tonge congratulated the IDF on its swift and generous response to the emergency in Haiti and said "To prevent allegations such as these – which have already been posted on YouTube – going any further, the IDF and the Israeli Medical Association should establish an independent inquiry immediately to clear the names of the team in Haiti."[28] On 12 February 2010, Liberal Democrat leader Nick Clegg removed her from her position as health spokeswoman in the House of Lords calling the comments "wrong, distasteful and provocative." He said she had apologised "unreservedly" for any offence she had caused.[29][30][31]

In 2012 Baroness Tonge appeared at an Israeli Apartheid Week talk at Middlesex University on 23 February 2012,[32] during which she said: "Beware Israel. Israel is not going to be there forever in its present form. One day, the United States of America will get sick of giving £70bn a year to Israel to support what I call America's aircraft carrier in the Middle East – that is Israel. One day, the American people are going to say to the Israel lobby in the USA: enough is enough. Israel will lose support and then they will reap what they have sown."[2] A party spokesman said: "Jenny Tonge does not speak for the party on Israel and Palestine. Her presence and comments at this event were extremely ill-advised and ill-judged."[33] Baroness Tonge resigned the Liberal Democrats whip position immediately after being given an ultimatum by party leader Nick Clegg to apologise or resign for her remarks.[2] She retains her position in the House of Lords as an independent Liberal Democrat.[34]

Tonge is chair of the "All Party Parliamentary Group on Population, Development and Reproductive Health" which in November 2012 issued a report on child marriage which urged the British government to act to stop child marriage in Britain and abroad.[35] In November 2012 Tonge spoke at a London rally at the Israeli Embassy against the Israeli blockade of the Gaza Strip and the November 2012 Israeli bombings of Gaza in retaliation for militant rocket attacks on Israel.[36]

She met her future husband, Dr Keith Tonge in 1959 at university[37] and married him on 23 May 1964 in Tipton. He was a consultant neuroradiologist formerly at St Thomas's Hospital in Lambeth and they have two surviving grown children and seven grandchildren. Dr Keith Tonge died on 5 July 2013.[38] She lives in Kew and also owns properties in Herault and Pimlico.[37][39][40]